In the .gitignore syntax there is an option where you can have a ! at the beginning of the file and it will un-ignore any previously ignored files.
So I can have the following:
logs/*
!logs/stuff.txt
And logs/stuff.txt will still be tracked.
What is the Mercurial .hgignore equivalent for this?
The equivalent is just adding the file. Once a file has been hg added the .hgignore file has absolutely no effect on how it's treated, so you ignore logs/* (be sure you're in glob mode, not regex mode) and then hg add logs/stuff.txt.